Wireless network communication systems that are based on the IEEE 802.11 (generally known as “WiFi”) standard operate using a variety of complex modulation methods. Communicating nodes, such as network clients and access points, utilize trial and error methods to choose a transmit modulation method that is deemed by each of the nodes to be most appropriate for the short term signal to noise environment. Modulation methods are typically changed several times a second based on error rates and receive signal strength measurements.
Table 1 shows the typical signal to noise ratio (SNR) required by each of the modulation methods employed in an 802.11a communications systems.
TABLE 1ModulationData RateRequired(Method)(Mbps)SNR (dB)BPSK ½68BPSK ¾99QPSK ½1211QPSK ¾181316-QAM ½241616-QAM ¾362064-QAM ⅔482464-QAM ¾5425In order to simplify the discussions Modulation Methods are often described by their associated Data Rate. For example, 64-QAM ¾ modulation is described as 54 Mbps modulation.
The noise component of the SNR is generally determined by the receiver noise figure. Most present day receivers operating at normal ambient temperatures have similar noise figures. The signal level component of the SNR is generally determined by transmit power, antenna gain, and path loss. The path loss component is commonly simplified to represent the range or distance between the two communicating network nodes. Of all the above mentioned factors that relate to SNR, generally the factors that are commonly thought of as representing communications quality are data rate, range. and Packet Error Rate (PER).
Many wireless networking environments such as, for example, auditoriums, lecture halls, hospitals and large offices often require a dense deployment of access points in order to provide adequate bandwidth to all wireless users. In these examples, range is a detrimental factor. Transmitting with too much power (range) may cause unintended same channel interference (co-channel interference and overlapping cells) as well as adjacent channel interference (interference on an adjacent channel) for nearby access points (increasing Packet Error Rates). Transmitting with too much power Lange) may also cause other 802.11 radios that belong to other cells to defer transmission per the CSMA/CA specification in the IEEE standard. These factors limit channel re-use, performance and access point density. In some environments it is desired to limit connection range in order to enhance security. Transmit power can be controlled to more precisely define the boundary of a coverage pattern to lower the possibility that clients outside a physical boundary, such as a wall, can connect to a particular access point. Additionally, there are situations where it is desired that the interference on the same channel be minimized between access points of neighboring users in houses, and other multi-tenant units where the density and the proximity of users force a higher degree of channel reuse.
There is a need for a system and method for controlling multi-modulation transmit range.